1. Field of the Invention
The present invention relates generally to a technical domain of a sliding element engaging a relative rotational motion. More particularly, the invention relates to a sliding element which reduces a friction coefficient on the sliding face and prevents a sealed fluid from leaking from the sliding face.
2. Description of the Related Art
There has been an increasing demand for a variety of machines nowadays which involve a high-speed rotary shaft as well as a high-pressure fluid therein such as in a compressor or turbine engine. These machines are more likely to rely on sliding elements in sliding portions thereof. The sliding elements employed therein need to have sliding faces which are capable of not only effecting seal against the sealed fluid but also exhibiting durability against a high-speed rotary motion. Therefore, from the viewpoint of a bearing functionality, the sliding element needs to bear a sliding face of high anti-wear ability and to retain lubricant fluid for decreasing the friction coefficient thereof. From the viewpoint of a seal ring of mechanical seal, the sliding element needs not only decrease the friction coefficient of the sliding face but also improve the seal performance thereof in order to effect seal against the sealed fluid under a high pressure.
There is a ring seal shown in FIG. 25 as a prior art 1 related to the present invention which is a sliding element of a mechanical seal having a double spiral groove thereon (for example, refer to FIG. 7 of U.S. Pat. No. 6,341,782B1). Ring seal 100 shown in FIG. 25 illustrates a frontal view of a seal face 101 thereof. The ring seal 100 assembled in pair constitutes a mechanical seal. A pair of the ring seals 100 in the mechanical seal consist of a rotary ring seal and a stationary ring seal. A relative sliding motion between the rotary ring seal 100 and the stationary ring seal provides seal against the sealed fluid which is fed from axially one side in the space defined between a rotary shaft 140 and a housing 150.
The seal face 101 of the ring seal 100 disposes an annular groove 115 near an inner circumferential surface 102. In addition, the seal face 101 disposes counter-flow pumping grooves 110 which take a spiral form extending inward from an outer circumferential surface 103 along the rotational direction of the rotary shaft 140 and communicating with the annular grooves 115. Likewise, pro-flow pumping grooves 111 are disposed thereon which also take a spiral form extending from the outer circumferential surface 103 and communicating with the counter-flow pumping grooves 110 on their way. Flows along the counter-flow pumping groove 110 and the pro-flow pumping groove 111, indicated by the pointing arrows A and B, respectively, meet at an intersection point 112. These counter-flow pumping grooves 110 and pro-flow pumping grooves 111 are arranged in an equally spaced manner on the seal surface 101. Namely, the counter-flow pumping grooves 110 form a plurality of spiral grooves on the seal face 101 and the pro-flow pumping grooves 111 coming from the outer circumferential surface 103 communicate with the counter-flow pumping grooves 110, which give a circulatory flow of the sealed fluid on the seal face 101. This ring seal 100 is fixed with a sleeve 130 which is fitted to the rotary shaft 140, thus rotating with the rotary shaft 140.
The ring seal 100 thus configured is brought into a non-contact state because of dynamic pressure induced by the spiral counter-flow pumping groove 110 when the seal ring 100 is subjected to a relative rotary movement between the seal face 101 and its opposing seal face, which decreases its seal capability. Also the circulation of the seal fluid over the entire area of the seal face 101 due to the fluid paths provided by the counter-flow pumping groove 110 and the pro-flow pumping groove 111, as indicated by the pointed arrows, implies presence of a possible leakage of the fluid through a dam portion 104 toward the opposite direction of the fluid. Such leaked fluid from the dam portion 104 toward the inner circumferential side is never fed back to the circulation path. This is another cause of damage to the seal capability.
There is a ring seal 200 as a second prior art 2 related to the present invention as shown in FIG. 26 (see FIG. 5 of U.S. Pat. No. 6,152,452, for example). This ring seal 200 is typically employed as a stationary ring seal or a rotary seal ring of a mechanical seal. Seal face 207 of the ring seal 200 disposes spiral grooves 201, 202 thereat. Out of the spiral grooves 201, 202, the outer circumferential region defined by two radii R3 and R4 includes high-pressure spiral grooves 201 which have a spiral form extending toward the outer circumferential direction. The inner circumferential region, on the other hand, defined by two radii R1 and R2 includes low-pressure spiral grooves 202 which also have a circumferential, spiral form. Disposed between the high-pressure spiral grooves 201 and the low-pressure spiral grooves 202 is a flat plain portion 206.
The high-pressure spiral groove 201 has a spiral form with a large line width and a very shallow depth when viewed from the top in order to bring the seal face 207 into a non-contact state by dragging in the sealed fluid. Therefore, when the ring seal 200 rotates, the fluid dragged in by the high-pressure spiral grooves 201 brings the seal face 207 into a non-contact state due to dynamic pressure generated thereat. It, however, has been known that the fluid leakage is likely to occur from the seal face 207 to the atmospheric side during the transition to the non-contact state. This deteriorates the seal capability of the seal face 207 as a matter of course. Even if the low-pressure spiral grooves 202 are introduced in the inner circumferential region of the seal face 207, the situation will not improve as long as the high-pressure spiral grooves 201 play a major role to bring the seal face 207 into a non-contact state by dragging in the sealed fluid. That is, the fluid leakage can hardly be prevented. In particular, the low-pressure spiral grooves 202 thus arranged, spiral curves defined in the region formed by the two radii R1 and R2, cannot provide the fluid with a significant pumping capability. More in particular, the fact that the adjacent low-pressure spiral grooves 202 lie side by side along the radial direction and the radius of the spiral grooves is more or less similar to the radius of the seal face 207, i.e., a small spiral angle with respect to the tangential line of rotation, only reveals a minor contribution of the adjacent low-pressure spiral grooves 202 to the pumping action against the fluid. In addition, a very small depth of groove such as no more than 1 micrometer imposes difficulties on accurate fabrication of the grooves and thereby affects resulting seal capability thereof.
As described above, it is apparently difficult to improve the seal capability with a non-contact face seal as disclosed in the prior art 2 wherein conventional high-pressure spiral grooves 201 are merely augmented by low-pressure spiral grooves 202 in the circumferential region of the seal face 207. In other words, because the seal face 207 serves as a non-contact seal, it can decrease the friction coefficient thereat but leaves a lot of room for improvement from the viewpoint of seal capability thereof.
The present invention is made to alleviate the above problems. A primary technical goal which this invention tries to achieve is not only to decrease the friction coefficient of sliding surfaces but also to improve seal capability thereof against sealed fluid by utilizing the fluid dragged onto the sliding surfaces. Another technical goal is to prevent heat generation on the sliding surfaces. Yet another technical goal is to enhance durability of the sliding surfaces by preventing their wear.